A hybrid vehicle is a type vehicle driven by efficiently combining two types of power sources, that is, a vehicle driven by an engine obtaining a torque by combusting fuel (e.g., fossil fuel such as gasoline) and an electric motor obtaining a torque by battery power. The hybrid vehicle is an environmentally-friendly vehicle that adopts an engine and an electric motor as an auxiliary power source to reduce exhaust gas and enhance fuel efficiency and research regarding the hybrid vehicle has been actively conducted to enhance fuel efficiency and develop environmentally-friendly products.
The hybrid vehicle generally uses an engine and a motor (e.g., a motor/generator) and uses the motor/generator having improved low slow torque characteristics at a low speed as a main power source and the engine having relatively high speed torque characteristics at a high speed as a main power source. Therefore, the hybrid vehicle stops an operation of the engine which uses fossil fuel at a low speed section and uses the motor/generator, and therefore exhibits improved fuel efficiency and reduced exhaust gas.
Further, to drive the vehicle, the hybrid powertrain is driven in various driving modes such as an electric vehicle (EV) mode which is a pure electric vehicle mode using the torque of the motor and a hybrid electric vehicle (HEV) mode which uses the torque of the motor as auxiliary power while using the torque of the engine as main power. In particular, the engine starts to perform a mode conversion from the EV mode into the HEV mode. Recently, a technology development for implementing a mode conversion between the power split mode and the parallel mode by subdividing the HEV mode into the power split mode and the parallel mode to increase power transfer efficiency has been conducted.